Getting Started

Kotlin Fundamentals

Course 1 · Chapter 1 · Getting Started

🚀 Getting Started with Kotlin

Kotlin is a modern, statically-typed language created by JetBrains that runs on the JVM (the same virtual machine Java runs on) and has been Android's officially preferred language since 2019. This chapter covers what Kotlin is and why it exists, how it compares to Java and JavaScript, and the absolute basics — declaring values, picking a type, and building strings.

☕ What Is Kotlin, and Why Learn It?

Kotlin is:

Statically typed — every value has a type, checked at compile time (like TypeScript, unlike plain JavaScript)
JVM-based — Kotlin source compiles to the same bytecode Java does, and runs on the same virtual machine
100% Java-interoperable — Kotlin code can call Java libraries directly, and vice versa
Android's preferred language — Google made Kotlin the recommended language for Android development in 2019, ahead of Java
Designed to fix Java's pain points — null safety, less boilerplate, and expression-oriented syntax are all direct responses to common Java frustrations

Kotlin vs Java vs JavaScript, at a Glance

Trait	JavaScript	Java	Kotlin
Typing	Dynamic	Static, verbose	Static, inferred
Where it runs	Browser / Node	JVM	JVM (also JS, Native)
Null handling	undefined / null, unchecked	null, unchecked (NPEs)	Null safety built into the type system
Semicolons	Optional (ASI)	Required	Optional
Entry point	Top-level script code	public static void main(String[] args)	fun main()

▶️ Your First Kotlin Program

Hello, Kotlin

// hello.kt
fun main() {
    println("Hello, Kotlin!")
}

Run it (from the command line, with the Kotlin compiler installed):

$ kotlinc hello.kt -include-runtime -d hello.jar
$ java -jar hello.jar
Hello, Kotlin!

In practice, almost nobody runs Kotlin this way day to day — IntelliJ IDEA (or Android Studio, which is built on it) compiles and runs Kotlin with a single click. The command-line path above is worth seeing once so fun main() doesn't feel like magic: it compiles to a .jar, which runs on the JVM exactly like compiled Java would.

Notice there's no class wrapping main, and no semicolon at the end of the println line — both required in Java, both optional in Kotlin. fun declares a function (short for function), and top-level functions — ones that live outside any class — are completely normal in Kotlin, unlike Java where every method must belong to a class.

📦 val vs var

Kotlin has exactly two ways to declare a value, and the choice is deliberate every time:

val name = "Philip"      // read-only reference — cannot be reassigned
var score = 0            // mutable — can be reassigned

score = 10                 // fine, score is a var
name = "Someone else"     // compile error — val cannot be reassigned

val — like JS const

A val is a read-only reference, assigned once. It maps directly onto JavaScript's const — same idea, same restriction. In Kotlin, val is the default choice; you reach for var only when you know a value needs to change.

var — like JS let

A var can be reassigned, the same as JavaScript's let. There's no Kotlin equivalent of JavaScript's old var keyword with its confusing function-scoping — Kotlin's var behaves like a normal block-scoped mutable variable.

Declaring a Mutable Value: Java vs Kotlin

Language	Immutable	Mutable
Java	final int score = 0;	int score = 0;
JavaScript	const score = 0;	let score = 0;
Kotlin	val score = 0	var score = 0

Java's default is mutable — you have to opt in to immutability with final, and most Java code doesn't bother. Kotlin flips that: val is the natural first choice, which pushes code toward fewer accidental mutations without any extra ceremony.

🔢 Basic Types & Type Inference

Kotlin's built-in types cover the same ground as Java's primitives, but without the primitive/object split:

val age: Int = 33
val price: Double = 19.99
val isReady: Boolean = true
val initial: Char = 'P'
val title: String = "Learning Blog"

// The type annotations above are optional — Kotlin infers them:
val inferredAge = 33       // inferred as Int
val inferredPrice = 19.99 // inferred as Double

vs Java: No Primitives

Java has a hard split between primitives (int, double, boolean) and their boxed object wrappers (Integer, Double, Boolean). Kotlin has just one type per concept — Int, Double, Boolean — and the compiler quietly uses the efficient primitive form under the hood wherever it can.

vs JavaScript: No Number-for-Everything

JavaScript has a single number type for all numeric values. Kotlin distinguishes Int (whole numbers) from Double (decimals) at compile time — mixing them without an explicit conversion is a compile error, not a silent runtime surprise.

Type inference means you rarely write the type annotation explicitly — Kotlin figures out Int, String, etc. from the value on the right-hand side, the same way TypeScript's let x = 5 infers number. The annotation is still useful (and sometimes required) when the type isn't obvious from context, such as an empty collection or a function parameter.

📝 String Templates

Kotlin builds strings with string templates — the direct equivalent of JavaScript's template literals, but with a different symbol:

val name = "Philip"
val age = 33

// Simple variable — just $name
println("Hello, $name!")                    // Hello, Philip!

// Expression — needs ${ } braces
println("Next year you'll be ${age + 1}")  // Next year you'll be 34

Template Syntax: JavaScript vs Kotlin

	JavaScript	Kotlin
Delimiter	Backticks: `...`	Double quotes: "..."
Simple variable	${name}	$name
Expression	${age + 1}	${age + 1}

The key difference: Kotlin string templates live inside ordinary double-quoted strings, not a special delimiter — so every String in Kotlin is automatically template-capable. For a single variable, the braces are optional ($name works fine); for anything more than a bare variable — a property access, a function call, an expression — the ${ } braces are required.

⚠ $ needs escaping if you mean it literally

Because $ triggers template interpolation, a literal dollar sign in a Kotlin string needs escaping: "Price: \$19.99". This is the same category of gotcha as needing to escape a literal backtick inside a JavaScript template literal.

💻 Coding Challenges

Challenge 1: val vs var in Practice

Write a Kotlin program with a val holding your name and a var holding a counter starting at 0. Increment the counter three times and print it after each increment. Then try (and comment out) reassigning the val — note the compiler error.

Goal: Build muscle memory for choosing val by default and reaching for var only when needed.

→ Solution

Challenge 2: Typed Values & Inference

Declare five vals — one each of type Int, Double, Boolean, Char, and String — first with explicit type annotations, then rewritten with type inference (no annotations). Print all five using one println per value.

Goal: Get comfortable with Kotlin's five basic types and when inference does the work for you.

→ Solution

Challenge 3: String Templates

Given a val for a product name and a var for its price (a Double), print a single sentence using a string template that includes the name (simple interpolation) and the price with 10% tax added (expression interpolation with ${ }).

Goal: Practice both forms of string template interpolation in one realistic sentence.

→ Solution

💡 Why This Course Starts on the JVM Track

Kotlin also compiles to JavaScript (Kotlin/JS) and native binaries (Kotlin/Native), but this course sticks to the JVM track throughout — it's the version used for Android development, which is the reason Kotlin is on the syllabus in the first place. Everything here transfers directly once the Android courses begin.

🎯 What's Next

Next chapter: Functions & Lambdas — default parameters, named arguments, single-expression functions, and Kotlin's lambda syntax.